Plant hormones are an unusual group of secondary plant constituents playing a regulatory role in plant growth and development. The regulating properties appear in course of the biosynthetic pathways and are followed by deactivation via catabolic processes. All these metabolic steps are in principle irreversible, except for some processes such as the formation of ester, glucoside and amide conjugates, where the free parent compound can be liberated by enzymatic hydrolysis. For each class of the plant hormones so-called ‘bound’ hormones have been found. In the early literature this term was applied to hormones bound to other low-molecular-weight substances or associated with macromolecules or cell structures irrespective of whether structural elucidation had been achieved.